The present invention relates to an apparatus suitable for transferring bituminous froth recovered from a hot water process for recovering bitumen from tar sands.
Tar sands, which are also known as oil and bituminous sands, are siliceous materials which are impregnated with a heavy petroleum. The largest and most important deposits of the sands are the Athabasca sands, found in northern Alberta, Canada. These sands underlay more than 13,000 square miles at a depth of 0 to 2,000 feet. Total recoverable reserves after extraction and processing are estimated at more than 300 billion barrels. Tar sands are primarily silica, having closely associated therewith an oil film which varies from about 5 to 21% by weight, with a typical content of 13 weight percent of the sand. The oil is quite viscous -- 6.degree. to 10.degree. API gravity -- and contains typically 4.5% sulfur and 38% aromatics. In addition to the oil and sand components, tar sands contain clay and silt in quantities of 1 to 10% by weight, in the form of a film around the sand grains.
Several basic extraction methods have been known for many years for the separation of oil from the sands. In the so-called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of a neutral salt in water, which salt is capable of acting as an electrolyte. The combined mass is then subjected to a pressure or gravity separation.
In the hot water method, as disclosed in Canadian Pat. No. 841,581 issued May 12, 1970, the bituminous sands are jetted with steam and mulled with a minor amount of hot water at temperatures of 170.degree. to 190.degree. F, and the resulting pulp is then dropped into a turbulent stream of circulating hot water and carried to a separation cell maintained at a temperature of about 185.degree. F. In the separation cell, sand settles to the bottom as tailings and oil rises to the top in the form of a froth. An aqueous middlings layer comprising clay and silt and some oil is formed between these layers. This basic process may be combined with a scavenger step for further treatment of the middlings layer obtained from the primary separation step to recover additional amounts of oil therefrom.
The bituminous froth produced by this process is recovered at a temperature in the range of about 140.degree. to 180.degree. F and normally has a specific gravity in the range of 0.85 to 0.95. Generally, the froth contains 30 to 70 weight percent air.
In recovering bituminous froth utilizing the process disclosed in Canadian Pat. No. 841,581 and the hot water separation cell disclosed in Canadian Pat. No. 882,667 issued Oct. 5, 1971, the froth is recovered in overflow launders disposed on the upper edge of the extraction cell. Thereafter, the froth flows by gravity into a collection vessel located near the separation cell below the level of the froth collection launders. Often, one collection vessel serves four or more separation cells at the same time to provide a central collection means for recovered froth. Froth from secondary scavenger steps as disclosed in the above-noted hot water process can also be collected in this same vessel. Thereafter, the froth is heated and transferred to a centrifuge zone or other means of demineralizing and dehydration. Normally, the froth is diluted with a liquid hydrocarbon before the demineralization and dehydration steps. Methods of accomplishing water and mineral removal from the froth are disclosed in Canadian Pat. Nos. 910,271 issued Sept. 19, 1972 and 918,091 issued Jan. 2, 1973.
The bituminous froth as recovered from the hot water extraction cell resembles a liquid foam with poor flow characteristics. The froth is difficult to pump and therefore must be treated to improve its liquid flow characteristics if it is to be processed in a reasonable manner. Canadian Pat. No. 630,710 issued Nov. 7, 1961 discloses that bituminous froth can be collected and transferred to a deaeration zone where it is heated with steam at subatmospheric pressures to remove air bubbles from the froth. This end can be accomplished by adding the froth to a steam heated oil bath maintained at subatmospheric pressure. The froth is therein diluted with oil and agitated to remove air bubbles from the froth. Although this method improves the froth, transferring the froth to the treatment apparatus disclosed nevertheless renders the process cumbersome. By the method of the present invention the processing of bituminous froth is improved.